In recent years, braking performance and driving performance on icy and snowy roads and wet road surfaces have been improved by providing multiple sipes, which are narrow grooves formed by incision, on the tread surface of a tire. Specifically, by providing sipes, an increase in the edge component is expected on icy and snowy roads, and enhanced drainage performance is expected on wet road surfaces. In either case, in addition to these aspects of performance, modifications to the shape of the sipes are being tested with regard to performance on dry road surfaces.
For example, with regard to suppressing the collapsing deformation of a block when introducing a sipe into the block, JP 2006-341816 A (PTL 1) proposes forming the sipe in the block to extend from the tread surface inwards in the tire radial direction while bending a plurality of times so as to have a zigzag-shape. It was discovered, however, that with such a sipe extending in a zigzag-shape, the location at which the blocks support each other from collapsing differs in the direction of sipe depth (tire radial direction) depending on the direction of input of force on the tread surface with respect to the land portion when the tire rolls. The effect of mutual support is thus easily dispersed.
To address this issue, JP 2013-244811 A (PTL 2) discloses a structure in which, among a plurality of bending points in a sipe that extends in a zigzag shape, a center region in the sipe depth direction is designated as a main-bend point, and two relatively large faces are provided in the center region in the sipe depth direction, thereby preventing the support locations of the sipe from being dispersed in the sipe depth direction depending on the direction of input of force to the block.
Since performance on icy and snowy roads is mainly emphasized for the tires disclosed in PTL 1 and PTL 2, these tires have a tread pattern of blocks defined by width direction grooves and circumferential grooves. When sipes are introduced into the blocks in such a tread pattern, the above-described problem of block collapse occurs. On the other hand, in a high-performance tire that guarantees performance up to high speeds, circumferential direction rigidity when driving at high speed is typically ensured by a rib-shaped land portion that is defined only by circumferential grooves and is continuous in the tread circumferential direction. For drainage performance, a tire having such a rib pattern may be provided with sipes extending in the width direction of the tread at an inclination relative to the tire equator. In other words, the tire is used with a designated rotation direction, with the sipes disposed to extend towards the tire rotation direction from the tread edge side to the tire equator side of the tread surface. In such a high-performance tire, the effect of the local reduction in land portion rigidity due to width direction sipes provided in the rib becomes a problem when a strong lateral force is applied to the tire, for example when changing lanes or turning at high speed on a circuit. Accordingly, in addition to drainage performance, there is a strong desire to improve turning performance at high speed with a tire having sipes in the rib-shaped land portion.
In other words, in the aforementioned tire that guarantees performance up to high speeds, there is a demand for using sipes to ensure drainage performance on wet road surfaces, while also suppressing a reduction in width direction rigidity of the tread on dry road surfaces and improving turning performance, in particular turning performance when driving at high speeds.